The perturbation of tryptophan fluorescence by phenylalanine to alanine mutations identifies the hydrophobic core in a subset of bacterial Ig-like domains

Crystal structure of a variable region segment of Leptospira host-interacting outer surface protein, LigA, reveals the orientation of Ig-like domains

Leptospiral immunoglobulin-like (Lig) protein family is a surface-exposed protein from the pathogenic Leptospira. The Lig protein family has been identified as an essential virulence factor of L. interrogan. One of the family members, LigA, contains 13 homologous tandem repeats of bacterial Ig-like (Big) domains in its extracellular portion. It is crucial in binding with the host's Extracellular matrices (ECM) and complement factors. However, its vital role in the invasion and evasion of pathogenic Leptospira, structural details, and domain organization of the extracellular portion of this protein are not explored thoroughly. Here, we described the first high-resolution crystal structure of a variable region segment (LigA8-9) of LigA at 1.87 Å resolution. The structure showed some remarkably distinctive aspects compared with the most closely related Immunoglobulin superfamily (IgSF) members. The structure illustrated the relative orientation of two domains and highlighted the role of the linker region in the domain orientation. We also observed an apparent electron density of Ca²⁺ ions coordinated with a proper interacting geometry within the protein. Molecular dynamic simulations demonstrated the involvement of a linker salt bridge in providing rigidity between the two domains. Our study proposes an overall arrangement of Ig-like domains in the LigA protein. The structural understanding of the extracellular portion of LigA and its interaction with the ECM provides insight into developing new therapeutics directed toward leptospirosis.

Leptospira Immunoglobulin-Like Protein B (LigB) Binds to Both the C-Terminal 23 Amino Acids of Fibrinogen αC Domain and Factor XIII: Insight into the Mechanism of LigB-Mediated Blockage of Fibrinogen α Chain Cross-Linking

The coagulation system provides a primitive but effective defense against hemorrhage. Soluble fibrinogen (Fg) monomers, composed of α, β and γ chains, are recruited to provide structural support for the formation of a hemostatic plug. Fg binds to platelets and is processed into a cross-linked fibrin polymer by the enzymatic clotting factors, thrombin and Factor XIII (FXIII). The newly formed fibrin-platelet clot can act as barrier to protect against pathogens from entering the bloodstream. Further, injuries caused by bacterial infections can be confined to the initial wound site. Many pathogenic bacteria have Fg-binding adhesins that can circumvent the coagulation pathway and allow the bacteria to sidestep containment. Fg expression is upregulated during lung infection providing an attachment surface for bacteria with the ability to produce Fg-binding adhesins. Fg binding by leptospira might play a crucial factor in Leptospira-associated pulmonary hemorrhage, the main factor contributing to lethality in severe cases of leptospirosis. The 12^th domain of Leptospira immunoglobulin-like protein B (LigB12), a leptospiral adhesin, interacts with the C-terminus of FgαC (FgαCC). In this study, the binding site for LigB12 was mapped to the final 23 amino acids at the C-terminal end of FgαCC (FgαCC8). The association of FgαCC8 with LigB12 (ELISA, K_D = 0.76 μM; SPR, K_D = 0.96 μM) was reduced by mutations of both charged residues (R608, R611 and H614 from FgαCC8; D1061 from LigB12) and hydrophobic residues (I613 from FgαCC8; F1054 and A1065 from LigB12). Additionally, LigB12 bound strongly to FXIII and also inhibited fibrin formation, suggesting that LigB can disrupt coagulation by suppressing FXIII activity. Here, the detailed binding mechanism of a leptospiral adhesin to a host hemostatic factor is characterized for the first time and should provide better insight into the pathogenesis of leptospirosis.

Leptospirosis, caused by pathogenic Leptospira spp., has been increasingly recognized as an emerging zoonosis worldwide. In human cases, clinical presentation can vary from a mild flu-like syndrome to severe multi-organ failure including hepatitis, nephritis and occasionally meningitis. Particularly, pulmonary hemorrhage has become one of the major factors leading to fatality. The host coagulation system normally can be activated to confine damage caused by bacteria. However, this spirochete has developed several virulence proteins to manipulate hemostatic factors including fibrinogen (Fg). Previously, we had observed that Leptospira immunoglobulin-like protein B (LigB) can bind to Fg and inhibit fibrin clot formation. In this study, the LigB binding site on fibrinogen was fine-mapped. The key amino acids contributing to this strong pathogen-host interaction were also identified. In addition, LigB bound to factor XIII and further interfered with the cross-linking of Fg. For the first time, a potential mechanism of leptospiral adhesin binding to fibrinogen was revealed, which should provide a better understanding of the pathogenesis of leptospirosis.

NMR solution structure of the terminal immunoglobulin-like domain from the leptospira host-interacting outer membrane protein, LigB

A number of surface proteins specific to pathogenic strains of Leptospira have been identified. The Lig protein family has shown promise as a marker in typing leptospiral isolates for pathogenesis and as an antigen in vaccines. We used NMR spectroscopy to solve the solution structure of the twelfth immunoglobulin-like (Ig-like) repeat domain from LigB (LigB-12). The fold is similar to that of other bacterial Ig-like domains and comprised mainly of β-strands that form a β-sandwich based on a Greek-key folding arrangement. Based on sequence analysis and conservation of structurally important residues, homology models for the other LigB Ig-like domains were generated. The set of LigB models illustrates the electrostatic differences between the domains as well as the possible interactions between neighboring domains. Understanding the structure of the extracellular portion of LigB and related proteins is important for developing diagnostic methods and new therapeutics directed toward leptospirosis.